The present invention relates to optical fiber telecommunications and more particularly to long-distance optical fiber telecommunications using soliton signals and wavelength division multiplexing (WDM).
The transmission of soliton pulses or solitons along a portion of an optical fiber having abnormal dispersion is a known phenomenon. Solitons are pulse signals of the sech2 waveform. With that type of pulse, the non-linearity in the corresponding portion of the fiber compensates for the dispersion of the optical signal, i.e. the dependence of the refractive index on the intensity of the optical signal is counterbalanced by the chromatic dispersion and vice versa. The transmission of the solitons is modelled in known manner by the non-linear Schrxc3x6dinger equation.
Various effects limit the transmission of such pulses, such as the jitter induced by the solitons interacting with the noise present in the transmission system, as described, for example in the article by J. P. Gordon and H. A. Haus, Optical Letters, vol. 11 n DEG 10 pages 665-667. That effect, referred to as the xe2x80x9cGordon-Haus effectxe2x80x9d, imposes a theoretical limit on the quality or the data rate of soliton transmission.
In order to exceed that limit, it is possible to use synchronous modulation of the solitons, by means of semiconductor modulators. That technique intrinsically limits the data rate of the soliton link because of the complexity and of the upper limit of the passband of the semiconductor modulators.
Another solution for exceeding the above-mentioned transmission limit consists in feeding solitons into an optical fiber transmission line in which dispersion is managed. That technique is described, for example, in the article by N. J. Smith et al. xe2x80x9cSoliton transmission using periodic dispersion compensationxe2x80x9d published in the xe2x80x9cJournal of Lightwave Technologyxe2x80x9d, Vol. 15, No. 10, October 1997. In such a dispersion-managed line, alternating segments of optical fiber respectively have normal dispersion and abnormal dispersion. The transmission line thus has low mean dispersion for which there exists a soliton-type pulse whose characteristics (duration, chirp, etc.) vary periodically. Such solitons that propagate in a dispersion-managed transmission line are also referred to as xe2x80x9cmanaged solitonsxe2x80x9d. Compared with a conventional soliton propagating in a line whose dispersion is not managed, a managed soliton offers the advantage that it can have higher energy, which makes it possible to increase considerably the total propagation distance. Furthermore, the dispersion management technique advantageously makes it possible to reduce non-linear effects, in particular in WDM transmission lines, such as, for example, cross phase modulation (XPM) or four-wave mixing (FWM).
In order to exceed the theoretical limit of transmission lines in which dispersion is not managed, it has also been proposed to use sliding guiding filters making it possible to control the jitter of the transmitted solitons, see, for example, EP-A-0 576 208. In that configuration, the transmission line is made opaque to noise while it is transparent to the soliton.
It might be thought that combining the dispersion-management technique with sliding guiding filters would make it possible to push back the transmission limits even further.
However, the Applicant has found, through digital simulations, that the opposite occurs. The use of sliding guiding filters in a dispersion-managed transmission line gives rise to amplitude fluctuations and time jitter that are considerable, which is unacceptable for a reliable optical fiber transmission system. Instead of improving transmission, said transmission is degraded considerably.
Furthermore, in order to increase the data rate of optical fiber transmission systems using soliton signals, it has also been proposed to use wavelength division multiplexing (WDM).
Wavelength division multiplexing, referred to as WDM below, consists in combining a plurality of modulated channels in the same fiber, each of the channels having a different carrier wavelength. Thus, the overall data rate of a transmission line is equal to the sum of the data rates of the various channels.
The present invention proposes a method and equipment for implementing the method that make it possible to push back further the transmission limit of the transmission technique using dispersion-managed optical fiber.
To this end, the invention provides a method of stabilizing managed optical solitons propagating in a dispersion-managed optical fiber transmission line, said method being characterized in that the following are performed periodically along the transmission line for each managed optical soliton:
a first step consisting in establishing coupling that is substantially linear between the energy and the spectrum width of the optical soliton; and
a second step consisting in setting the spectrum width by filtering, and, by means of the substantially linear coupling, in setting the energy of the soliton resulting from the first step, the center frequency of the filtering being substantially equal to the center frequency of the optical soliton.
The method of the invention may also have one or more of the following characteristics:
during the first step, each managed optical soliton is converted into a pure Schrxc3x6dinger soliton by amplifying it and by feeding it into an optical fiber having dispersion matched to the amplitude of the amplified soliton, thereby establishing coupling that is substantially linear between the energy and the spectrum width of the optical soliton;
during the first step, each managed optical soliton is amplified and then fed into a fiber that is highly non-linear compared with the transmission line, and that is not very dispersive, the energy of the amplified solitons being sufficient to cause non-linear optical Kerr effects when the amplified solitons propagate in said fiber that is highly non linear and not very dispersive, thereby establishing coupling that is substantially linear between the energy and the spectrum width of the optical soliton;
for managed solitons with chirp, after the second step, a third step is performed that consists in re-establishing the chirp of the optical soliton as it was before the first step; and
when the optical transmission line is a WDM optical fiber line, and when each managed optical soliton propagates in an associated channel of a plurality of multiplexed channels, a demultiplexing step is performed prior to said first step, and a multiplexing step is performed after said last step of the method, and the steps of the method are performed on each channel individually.
The invention also provides equipment for implementing the method as defined above, said equipment being characterized in that it comprises:
first means for establishing coupling that is substantially linear between the energy and the spectrum width of the optical soliton; and
second means for setting the spectrum width by filtering, and, by means of the substantially linear coupling, for setting the energy of the soliton at the outlet of the first means, the center frequency of the filtering being substantially equal to the center frequency of the optical soliton.
The equipment of the invention may also have one or more of the following characteristics:
the first means for establishing coupling that is substantially linear between the energy and the spectrum width of the optical soliton comprise, disposed in line, an amplifier and an optical fiber having dispersion matched to the amplitude of the optical soliton so as to convert said soliton into a pure Schrxc3x6dinger soliton;
the first means for establishing coupling that is substantially linear between the energy and the spectrum width of the optical soliton comprise, disposed in line, an amplifier and a fiber that is highly non-linear compared with the transmission line, and that is not very dispersive, the energy of the amplified solitons being sufficient to cause non-linear optical Kerr effects when the amplified solitons propagate in said fiber that is highly non linear and not very dispersive;
for managed solitons with chirp, downstream from said second means, the equipment further comprises third means for re-establishing the chirp of the optical soliton as it was upstream from said first means; and
when the optical transmission line is a WDM optical fiber line, and when each managed optical soliton propagates in an associated channel of a plurality of multiplexed channels, the equipment further comprises demultiplexing means disposed upstream from said first means, and multiplexing means disposed downstream from said second means or from said third means.
Other characteristics and advantages of the invention appear from the following description given by way of non-limiting example and with reference to the accompanying drawings, in which: